Metal hydrides and their formers have been under study for some years as hydrogen absorbers. It should be understood that metal hydride compounds may be divided into three general categories: saline or ionic, metallic, and covalent. This classification is based on the predominant character of the hydrogen bond. In the ionic hydrides, the crystal lattice consists of metal cations and hydrogen anions. Such hydrides are formed by the direct reaction of hydrogen with the alkali and alkaline earth metals and magnesium. Hydrides of these metals are quite similar to their halide analogues both in structure and properties. With the exception of magnesium, the ionic hydrides are too stable to be considered attractive hydrogen storage media. The covalent hydrides are formed by Be and many of the B group metals of the periodic table. These hydrides can be solid, liquid or gaseous. None of the covalent hydrides can be formed by the direct and reversible reaction of the solid metal with hydrogen and none of these hydrides can be used as hydrogen storage media.
The metallic hydrides exhibit typical metallic properties having a metallic appearance and high thermal and electrical conductivities. They can be formed by the direct and reversible reaction of hydrogen with most of the transition metals (periodic table Groups IIIA to VIIIA), including the lanthanide and actinide series. It is this group of metal hydrides that are useful as hydrogen absorbers. Their large capacity for hydrogen storage coupled with their ready release of hydrogen at moderate temperatures and pressures and their ability to undergo many cycles of absorption and desorption with little decrease in capacity make them potentially useful in a variety of industrial applications, including hydrogen storage in energy conversion cycles, chemical heat pumps and compressors, hydrogen purification, and hydrogen isotope separation.
Two properties of metal hydrides have been found to present significant problems in the design of pressure vessels or reactors containing metal hydrides, usually in granular or powder form, in which the absorption and desorption of hydrogen is alternately carried out: (1) the tendency of the hydride particles to fragment during successive absorption-desorption cycles, resulting in the production of undesirable fine powders after only a few cycles, with appreciable changes in bed volume and (2) the appreciable heats of hydriding (absorption) and dehydriding (desorption).
In conventional gas-solid beds, the fragmentation property requires special designs to deal with the resulting settling and compaction and the consequent deformation and hydrogen flow distribution problems in the containment vessels. Also, in gas-solid beds, the large heats of hydriding and dehydriding require complicated facilities for the removal and addition of heat, respectively, and/or long absorption and desorption times in order to avoid excessive temperature gradients within the bed, with consequent imcomplete absorptions and desorptions and, thus, reduced sorbent utilization.
Typical reversible metal hydride systems of the prior art are dry bed systems such as set forth in U.S. Pat. No. 3,508,414--Apr. 28, 1970--R. H. Wiswall and J. J. Reilly. Such systems involve contact of hydrogen with an aggregate of metal hydride particles. Severe constraints have been imposed in overall system design with these systems becasue of parameters that seriously limit heat transfer, separation of gas and solids, volumetric changes from expansion and contraction, immobility of the beds, and poisoning of active surfaces by contaminants.
One remedy for these problems has been proposed by Rudman, et al., "Hydrogen Separation From Gas Mixtures Using LaNi.sub.5 Pellets", J. Less Common Metals, 89, 437 (1983). In this approach, pelleted forms of a given hydride former in admixture with a metal are used in an attempt to provide a thermal ballast and to decrease fragmentation. However, the use of the pellets increases the cost and decreases the efficiency of the process while failing to deal significantly with the heat and fragmentation problems.
Metals and metal alloys known to form reversible hydrides for reversibly capturing hydrogen include titanium alloys as set forth in U.S. Pat. No. 4,075,312--Feb. 21, 1978--J. Tanaka, et al., and lanthanum alloys disclosed in U.S. Pat. No. 4,142,300--Mar. 6, 1979--D. M. Gruen, et al. Other alloys are available as shown in U.S. Pat. No. 4,200,623--Apr. 29, 1980--A. Muller, et al. Elemental metals known to form metal hydrides are described in "Metal Hydrides" by W. M. Mueller, J. P. Blackledge and G. G. Libowitz, Academic Press, N.Y. 1968.
The present invention overcomes the problems of the prior art processes by using a slurry of metal hydride particles suspended in an inert liquid or solvent. In a slurry reactor, particle fragmentation does not produce vessel deformation. Problems such as restricted access of hydrogen to particles due to compacting in gas-solid systems are eliminated in well-agitated gas-liquid-solid slurry systems. In addition, temperature gradients within slurry reactors become negligible compared to those in gas-solid systems.